Rheology modifiers are used generally to adjust or modify the rheological properties of aqueous compositions. Such properties include, without limitation, viscosity, flow rate, stability to viscosity change over time, and the ability to suspend particles in such aqueous compositions. The particular type of modifier used will depend on the particular aqueous composition to be modified and on the particular end-use of that modified aqueous composition. Examples of conventional rheology modifiers include thickeners such as cellulosic derivatives, polyvinyl alcohol, sodium polyacrylate, and other water-soluble macromolecules, and copolymeric emulsions in which monomers with acid groups have been introduced onto the main chain. Such thickeners are used widely in fiber treatment and adhesives.
It has been reported that when thickeners such as cellulosic derivatives and polyvinyl alcohol are mixed with aqueous emulsions, the thickened emulsion tends to exhibit poor stability to viscosity change over time. The cellulosics are said to result in a substantial decline in viscosity over time. It also has been reported that large quantities of polyvinyl alcohol are required in order to thicken aqueous emulsions. When such thickened aqueous emulsions are used in, for example, adhesives and coatings, the high levels of polyvinyl alcohol result in a loss of adhesive and/or cohesive properties as well as a loss in water resistance in the films formed therefrom.
Another class of rheology modifiers known to thicken aqueous emulsions is one typically referred to as associative modifiers. Such associative modifiers are reported in U.S. Pat. Nos. 4,743,698, 4,600,761, RE 33,156, 4,792,343, 4,384,096, 3,657,175, 5,102,936 and 5,294,692. As noted, these thickeners become effective upon the addition of base, thereby raising the pH of the thickened composition to alkaline, but the thickeners do not thicken aqueous compositions having acidic pH.
Other rheology modifiers which are "activated" by the addition of acid to aqueous compositions which contain the modifiers also have been reported. As reported, emulsions are prepared via free-radical emulsion polymerization utilizing colloidal stabilizers. The emulsions are mixed with the composition to be thickened and then acid is added to the mix, thereby lowering the pH of the system to 6.5 to 0.5. These thickeners are reported to be effective at thickening certain acidic aqueous compositions, but are not effective at thickening aqueous compositions having basic pH.
It would be desirable to develop a rheology modifier which is stable to change in viscosity and phase separation over time, which does not detrimentally affect film properties such as adhesive/cohesive properties and water resistance, and which advantageously may be used to thicken both acidic and basic aqueous compositions.